Just seems to me that it's way too soon to be thinking post-credits for IM3. Also doubt that they would be tinkering with "The Avengers" at this late date. That's why I'm guessing that it's the post-credit scene for "Avengers." Maybe they waited for all the guys to be in town to shoot it. Much easier to schedule that way.
Of course, I could be miles off base with my theory.